1. Field of the Invention
The invention relates to hybrid electric vehicles and a method for estimating vehicle wheel torque.
2. Background Art
Unlike pure electric vehicles that use a battery as a power source for a motor in a power flow path to traction wheels, a hybrid electric vehicle has an engine (typically an internal combustion engine) and a high voltage motor for powering the vehicle. A known powertrain configuration for a hybrid electric vehicle consists of two power sources that are connected to the vehicle traction wheels through a planetary gearset. A first power source in this powertrain configuration is a combination of an engine, a generator and a planetary gearset. A second power source comprises an electric drive system including a motor, a generator and a battery subsystem. The battery subsystem acts as an energy storing device for the generator and the motor.
In the case of the first power source, the engine speed can be decoupled from the vehicle speed since the generator acts as a torque reaction element for a reaction gear of the planetary gearset. This results in both a mechanical torque flow path and an electromechanical torque flow path, which function in tandem to deliver driving torque to the vehicle traction wheels. The generator reaction torque effects engine speed control as it provides a reaction torque in the torque flow path from the engine. This operating mode commonly is referred to as a non-parallel operating mode. If the generator is braked, the reaction element of the gearset also becomes braked, which establishes a fully mechanical power flow path from the engine to the traction wheels through the gearset. This is referred to as a parallel operating mode. An example of a powertrain configuration of this type can be seen by referring to co-pending U.S. patent application Ser. No. 10/248,886, filed Feb. 27, 2003, now U.S. Pat. No. 6,991,053, dated Jan. 31, 2006. This co-pending patent application is assigned to the assignee of the present invention.
In the powertrain configuration disclosed in the co-pending patent application, torque is delivered through the powertrain for forward motion only in the case of the first power source. In the case of the second power source, the electric motor draws electric power from the battery and provides driving torque independently of the engine in both forward and reverse drive. In this operating mode, the generator, using battery power, can drive against a one-way clutch on the engine output shaft to propel the vehicle forward.
A control system is used to effect integration of the two power sources so that they work together seamlessly to meet the driver's demand for power at the traction wheels without exceeding the limits of the battery subsystem. This is accomplished in the powertrain of the co-pending patent application by coordinating the control of the two power sources. Under normal powertrain operating conditions, a vehicle system controller interprets a driver demand for power, which may be an acceleration or deceleration demand, and then determines a wheel torque demand based on driver demand and powertrain limits. The vehicle system controller also will determine when and how much torque each power source must provide to meet the driver's demand and to achieve specified vehicle performance, such as fuel economy, emissions, driveability, etc. The vehicle system controller can control the engine operating speed for each torque demand so that an efficient operating point on the speed-torque engine characteristic curve will be established.
A control system of the type discussed in the preceding paragraphs requires a so-called drive-by-wire control system as the two power sources cooperate seamlessly to achieve optimal performance and efficiency. Such a drive-by-wire system requires a torque monitor strategy to ensure that the control system wheel torque demand and the actual powertrain torque output are within a predefined range so that unintended vehicle acceleration will be avoided.
U.S. Pat. No. 5,452,207, which is owned by the assignee of the present invention, discloses a torque estimation method based on a vehicle dynamics model, a torque converter model and an engine torque model. Estimates of torque are obtained from at least two of the models. The torque estimates are weighted according to a predefined strategy and then transferred to a controller for developing torque estimates based on the weighted individual torque estimates.
A wheel torque estimation strategy is disclosed also in U.S. Pat. No. 5,751,579, which also is owned by the assignee of the present invention. It provides an estimate of wheel torque based upon engine combustion torque. The estimated torque is proportional to engine acceleration and engine powertrain mass.